The Gigabit Passive Optical Network (GPON) is an important technical branch of the Passive Optical Network (PON) family, and similar to the other PON techniques, the GPON is also a passive optical access technique using a topology structure of point to multipoint.
The topology structure of the GPON system is as shown in FIG. 1, wherein the GPON is composed by an Optical Line Terminal (OLT) at the central office side, an Optical Network Unit (ONU) at the subscriber side, and an Optical Distribution Network (ODN), and generally uses the network structure of point to multipoint. The ODN is composed by passive optical components, such as single mode fibers, one or more optical splitters and optical connectors, and the like, to provide an optical transmission medium for the physical connection between the OLT and the ONU.
In the GPON system, the data transmission in downstream (from the OLT to the ONU) adopts the way of broadcast. Each ONU receives all of the downstream frames in the network, and then obtains the frame belonging to the ONU according to the ONU Identity (ONU-ID), GPON Encapsulated Method-Port Identity (GEM-Port ID), and Allocation Identity (Allocation-ID). However, for the data transmission in the upstream (from the ONU to the OLT), since various ONUs should share the transmission medium, various ONUs should transmit the upstream data within the slot allocated for this ONU. Since the distances between various ONUs and the OLT are different, in order to avoid that the upstream data sent by the various ONU arrives at the OLT at the same time, the OLT needs to perform ranging on each ONU and send the equalization delay obtained according to the ranging result to the corresponding ONU. The ONU adjusts the clock used for sending the data according to the equalization delay sent by the OLT, that is to say, when the ONU is intend to send the upstream frame, the ONU needs to send the upstream frame after delaying own equalization delay, thereby implementing the synchronization of all of the ONU upstream transmission.
The ONU has 7 states, which are respectively an initial state, a standby state, a serial number state, a ranging state, an operation state, a POPUP state and an emergency stop state. In the related art, the OLT performs ranging on the ONU that is in the ranging state, which mainly comprises following steps of: the OLT sending a ranging request to the ONU and opening a quiet window, and configuring the open time of the quiet window. During the open time of the quiet window, the ONUs that are in the operation state all do not send the upstream data, so as to avoid the conflict with the message sent by the ONU that is in the ranging state; the ONU which receives the ranging request sends the own serial number information to the OLT; the OLT performs ranging on the ONU, and sends the ranging result to the ONU by the Ranging-time message; the ONU receives the Ranging-time message and then gets into the operation state and implements the synchronization of the upstream transmission. Since the OLT does not know the position of the ONU on which ranging is to be performed, the OLT opens the quiet window for the ONU on which ranging is to be performed according to the nearest distance to the ONU and the furthest distance to the ONU that the OLT supports. However, too long open time of the quiet window used for the ranging will interrupt the normal service of the ONU that is in the operation state, thereby affecting the efficiency of the upstream transmission.